CT scanning is a process for imaging the internal structure of a patient. In conventional CT scanning, a beam of x-rays is projected through the patient and its attenuation is measured. At the same time, the apparatus is rotated about an axis passing longitudinally through the patient. Thus, data is acquired as to the attenuation of the beam in each direction in the plane in which rotation takes place. From this data, the internal structure of the patient on that plane can be computed. The patient or apparatus is then indexed along the axis and a further plane (known as a ‘slice’) is then investigated. A three dimensional image of the patient can then be constructed from the various slices. One problem is that over the time required to acquire the necessary slices, the patient is not motionless. Gross motor movement can be avoided by suitable instruction to the patient, but even so each slice is acquired at a different phase of the breathing cycle. This results in a beating artifact due to the different frequency of breathing and slice acquisition.
Two ways have been used to solve this problem. On is to trigger the CT on a particular phase of the patients breathing. This is termed ‘respiration gated CT’ and implies that one CT slice is acquired for every breath. This means that it takes a long time to acquire a complete volume of data.
Another technique is to monitor the phase of the patients breathing whilst acquiring CT slices continuously. Once the data is acquired, slices that have comparable breathing phase are selected from the complete set and these are then used to visualize the volume. This has the advantage that any phase can be selected retrospectively and therefore the effect of breathing can be studied. This is termed ‘respiration correlated CT’.
Conventional CT scans have the disadvantage that the resolution along the axis is poor since it corresponds to the slice thickness. It is theoretically straightforward to increase this, but doing so results in a correspondingly longer acquisition time, or the need to rotate the apparatus correspondingly faster. Both options also give rise to an attendant reduction in contrast in the measured beam. Accordingly, ‘cone beam CT’ methods have been developed, in which a conical beam of radiation is directed at the patient and a two-dimensional image acquired via a flat panel detector. This apparatus is then rotated around the patient axis and a three-dimensional image is reconstructed from the set of two-dimensional images. As the individual slices are eliminated, there is the same resolution in all directions of the image. Likewise, as there are no slices the above-mentioned breathing artifact is absent since there can be no variation in patient position between slices.